Mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, and other portable electronic devices utilize SSL devices (e.g., light emitting diodes (LEDs)) for background illumination. SSL devices are also used for signage, indoor lighting, outdoor lighting, and other types of general illumination. FIGS. 1A and 1B are cross-sectional and plan views of a conventional SSL device 10 with lateral contacts, respectively. As shown in FIG. 1A, the SSL device 10 includes a substrate 12 carrying an LED structure 11 having N-type gallium nitride (GaN) 14, GaN/indium gallium nitride (InGaN) multiple quantum wells (“MQWs”) 16, and P-type GaN 18. Referring to FIGS. 1A and 1B, the SSL device 10 also includes a first contact 20 on the N-type GaN 14 and a second contact 22 (shown with a transparent current spreading material 29) on the P-type GaN 18. The first and second contacts 20 and 22 also include first and second contact fingers 20a, 20b, 22a, and 22b (FIG. 1B), respectively. FIGS. 2A and 2B are cross-sectional and plan views of another conventional SSL device 10′ with vertical contacts, respectively. The first contact 20 includes a plurality of conductive fingers 21 (three are shown for illustration purposes) coupled to one another by a cross member 23. The second contact 22 includes a reflective and conductive material (e.g., aluminum).
In operation, a continuous or pulsed electrical voltage is applied between the first and second contacts 20 and 22. In response, an electrical current flows from the first contact 20, through the P-type GaN 18, the GaN/InGaN MQWs 16, and the N-type GaN 14, to the second contact 22. The GaN/InGaN MQWs 16 then convert a portion of the electrical energy into light. The generated light is emitted from the SSL devices 10 and 10′ for illumination, signage, and/or other suitable purposes.
It is believed that SSL devices can operate at peak efficiencies when current densities are at or near a threshold level in the SSL devices. However, as discussed in more detail later, the current densities through different regions of the GaN/InGaN MQWs 16 in the SSL devices 10 and 10′ can be spatially non-uniform. For example, as shown in FIG. 1B, certain regions (e.g., regions II and III) of the GaN/InGaN MQWs 16 of the SSL device 10 may operate at lower current densities than other regions (e.g., region I) of the GaN/InGaN MQWs 16. In another example, the SSL device 10′ can have varying current densities based on the location of the conductive fingers 21. Such spatially varying current densities can reduce the overall operating efficiency of the SSL devices 10 and 10′. Accordingly, several improvements to balance regional current densities in SSL devices may be desirable.